Reinforced leadframe to substrate attachment

ABSTRACT

A method and apparatus for a reinforced leadframe to substrate attachment in a semiconductor assembly. In one embodiment, a printed circuit board having a plurality of electrically coupled electrical contact regions and wire bond areas formed thereon has a leadframe attached thereto such that each of the bonding fingers of the leadframe is coupled to a respective electrical contact region on the printed circuit board. A ribbon of B-staged epoxy is disposed on the leadframe such that said leadframe is disposed between the ribbon of B-staged epoxy and the printed circuit board. An integrated-circuit die is mounted on the printed circuit board with the bonding fingers of the leadframe peripherally surrounding the integrated circuit die. The bonding pads on the integrated-circuit die are electrically coupled to respective wire bond areas on the printed circuit board. By placing the ribbon of B-staged epoxy over the leadframe, the connection between each bonding finger of the leadframe and the respective electrical contact region is reinforced. Thus, the present claimed invention reduces separation between each bonding finger of the leadframe and the respective electrical contact region of the printed circuit board thereby providing improved reliability and yield in such semiconductor assemblies.

This Application is a Divisional of U.S. patent application Ser. No.08/885,322 filed Jun. 30, 1997, now U.S. Pat. No. 5,905,300, issued May18, 1999, which is a File Wrapper Continuation of Ser. No. 08/222,204filed Mar. 31, 1994 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to semiconductor manufacturing techniques.Specifically, the present invention relates to reinforced attachment ofa leadframe to a substrate.

2. Prior Art

Leadframes are commonly attached to substrates to form aleadframe/substrate assembly. Often, the leadframe is attached to thesubstrate with the smaller inner tips of the bonding fingers attached tothe substrate. An integrated-circuit die is then attached to thesubstrate within the central opening contained within the leadframe.Bonding pads on the integrated-circuit die are electrically connected torespective bonding fingers to form a semiconductor assembly.

However, during subsequent handling of the leadframe, the bondingfingers of the leadframe may lift or detach from the substrate. If suchdetachment occurs the reliability of the semiconductor assembly isseverely degraded. The degradation of reliability may occur as a resultof compromised electrical connection between the integrated-circuit dieand the detached bonding fingers, shorting due to contact between thedetached bonding fingers and adjacent bonding fingers, or for numerousother reasons.

Additionally, the problem of bonding finger detachment from thesubstrate is compounded as ultra-fine pitch leadframes are developed.That is, with even greater numbers of bonding fingers per leadframe andwith the bonding fingers become correspondingly smaller or thinner, theintegrity of the attachment of the leadframe bonding fingers to thesubstrate is of greater concern.

In seeking to reinforce the leadframe to substrate attachment, it isdesired to keep the cost of the assembly to a minimum, and insure thatany reinforcement means are compatible with standard subsequentprocessing steps.

Thus, the need has arisen for means to reinforce leadframe to substrateattachment which does not substantially increase the cost of theassembly and which is compatible with standard subsequent processingsteps.

SUMMARY OF THE INVENTION

It is therefore one object of the present invention to provide areinforced leadframe to substrate attachment which does notsubstantially increase the cost of the assembly and which is compatiblewith standard subsequent processing steps. Therefore, the presentinvention provides, in one embodiment, a layer of adhesive which isplaced on a leadframe after the leadframe has been attached by thermalcompression to a substrate. The layer of adhesive such as, for example,epoxy, is placed over the leadframe such that the leadframe is disposedbetween the substrate and the layer of epoxy. After curing, the epoxyserves to reinforce the thermal compression created bond between theleadframe and the substrate. As a result, the reliability and yield ofthe package is enhanced. Furthermore, the epoxy does not substantiallyincrease the cost of the leadframe/substrate attachment, and the layerof epoxy is compatible with standard subsequent processing steps.

In another embodiment of the present claimed invention, a layer ofadhesive is placed on a leadframe after the leadframe has been attachedby thermal compression to a printed circuit board. The printed circuitboard has a plurality of electrically connected wire bond areas andelectrical contact regions formed thereon. The leadframe is attached tothe printed circuit board such that each of the bonding fingers iscoupled to a respective electrical contact region on the printed circuitboard. Next, a layer of adhesive such as, for example, a ribbon ofB-staged epoxy is placed over the leadframe such that the leadframe isdisposed between the printed circuit board and the ribbon of B-stagedepoxy. After curing, the ribbon of B-staged epoxy serves to reinforcethe thermal compression created bond between the leadframe and theprinted circuit board. As a result the reliability and yield of thepackage is enhanced.

Thus, the present claimed invention provides a reinforced leadframe tosubstrate attachment which does not substantially increase the cost ofthe assembly and which is compatible with standard subsequent processingsteps.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention:

FIG. 1 is a perspective view of a portion of a Prior Art conventionalleadframe mounted on a substrate.

FIG. 2 is a perspective view of one embodiment of the invention in whicha leadframe/substrate assembly has a layer of adhesive disposedthereover in accordance with the present invention.

FIG. 3 is a perspective view of another embodiment of the invention inwhich a leadframe/substrate assembly has a layer of adhesive disposedover a large portion thereof in accordance with the present invention.

FIG. 4 is a perspective view of a portion of a Prior Art printed circuitboard having electrical contact regions and wire bond areas formedthereon.

FIG. 5 is a perspective view of another embodiment of the invention inwhich a leadframe/printed circuit board assembly has a ribbon ofB-staged epoxy disposed thereover, (the ribbon of B-staged epoxy isshown in a partial cutaway view for clarity) in accordance with thepresent invention.

FIG. 6 is a perspective view of another embodiment of the invention inwhich a leadframe/printed circuit board assembly has a ribbon ofB-staged epoxy disposed over a large portion thereof (the ribbon ofB-staged epoxy is shown in a partial cutaway view for clarity) inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the appended claims.

With reference now to Prior Art FIG. 1, a perspective view of a portionof a conventional leadframe 10 mounted on a substrate is shown.Leadframe 10 is attached to substrate 12 using, for example, thermalcompression with bonding fingers, typically shown as 14, extendinginwardly and peripherally surrounding a centrally located opening inleadframe 10. An integrated-circuit die 16 is attached to substrate 12within the centrally located opening of leadframe 10. Bonding pads,typically shown as 18, on integrated-circuit die 16 are electricallyconnected to bonding fingers 14 using, for example, bonding wires,typically shown as 20. However, during subsequent handling, or simply asthe result of weak bonds between leadframe 10 and substrate 12, bondingfingers 14 may detach or lift from substrate 12.

With reference still to Prior Art FIG. 1, when such detachment occursbetween bonding fingers 14 and substrate 12, the reliability of thesemiconductor assembly is severely degraded. The degradation ofreliability may occur as a result of intermittent electrical connectionbetween integrated-circuit die 16 and the detached bonding fingers,shorting due to contact between the detached bonding fingers andadjacent bonding fingers, or for numerous other reasons.

Referring now to FIG. 2, perspective view of a leadframe/substrateassembly having a layer of adhesive disposed thereover is shown. Aleadframe 30 is disposed on the top surface of a substrate 32. In thepresent embodiment, substrate 32 is a thermally-conductive,electrically-insulated substrate. Although such a substrate is used inthe present embodiment, the present invention is also well suited to theuse of other substrates well known in the art including but not limitedto a printed circuit board. After the attachment of leadframe 30 tosubstrate 32, an inexpensive layer of adhesive 34 is placed overleadframe 30 and substrate 32. In the present embodiment, layer 34 isformed of epoxy, although the present claimed invention is well suitedto the use of numerous other types of adhesive well known in the art,including but not limited to a ribbon of B-staged epoxy. After a curingprocess, epoxy layer 34 reinforces the bond between leadframe 30 andsubstrate 32. After curing of epoxy layer 34, an integrated-circuit die,not shown, is mounted on substrate 32 and electrical connection is madefrom bonding pads of the integrated-circuit die to respective bondingfingers, typically shown as 36, of leadframe 30. By placing layer 34 onleadframe 30, detachment of bonding fingers 36 from substrate 32 isprevented, thereby improving the reliability of the semiconductorassembly.

With reference now to FIG. 3 a perspective view of another embodiment ofthe present claimed invention is shown. Although epoxy layer 34 isdisposed over leadframe 30 only above the point at which the inner endsof bonding fingers 36 are attached to substrate 32 in FIG. 2, thepresent claimed invention is well suited to other configurations ofepoxy layer 34. As shown in the embodiment of FIG. 3, epoxy layer 34 isplaced over the entire surface of leadframe 30, hidden, and substrate 32except for the portion of substrate 32 which is exposed through theopening centrally located within leadframe 30. In such a configuration,because layer of epoxy 34 is chemically compatible with the plasticmaterial used in subsequent processes to encapsulate the semiconductorassembly, the adhesion of mold compound to the semiconductor assembly isenhanced. Thus, the present claimed invention provides a reinforcedleadframe to substrate attachment which does not substantially increasethe cost of the assembly and which is compatible with standardsubsequent processing steps.

With reference next to Prior Art FIG. 4, a perspective view of a portionof a conventional printed circuit board 40 is shown. Printed circuitboard 40 has electrical contact regions, typically shown as 42, and wirebond areas, typically shown as 44, formed thereon. Electrical contactregions 42 and wire bond areas 44 are electrically connected throughtraces in printed circuit board 40. Electrical contact regions 42 andwire bond areas 44 are formed on printed circuit board 40 to accommodatethe placement of a leadframe thereon. Typically, a leadframe, not shownis attached to printed circuit board 40 with the inner tips of thebonding fingers of the leadframe attached to respective electricalcontact regions 42. After an integrated-circuit die, not shown, ismounted to printed circuit board 40 within the opening centrally locatedwithin the leadframe, bonding pads on the integrated-circuit die areelectrically connected to wire bond areas 44 of printed circuit board 40using, for example, bonding wires, not shown.

With reference now to FIG. 5, another embodiment of the presentinvention is shown. As shown in FIG. 5, a leadframe 56 is attached toprinted circuit board 50 using, for example, thermal compression suchthat the inner tips of bonding fingers, typically shown as 58 areconnected to respective electrical contact regions, typically shown as52. Although a printed circuit board is used in the present embodiment,the present invention is also well suited to numerous other types ofsubstrates including, but not limited to, thermally-conductive,electrically insulated substrates.

With reference still to FIG. 5, an inexpensive ribbon of B-staged epoxy60 is placed over leadframe 56 and printed circuit board 50 above thepoint where the inner tips of bonding fingers 58 are connected toelectrical contact regions 52 of printed circuit board 50. In thepresent embodiment, a ribbon of B-staged epoxy 60 is used, although thepresent claimed invention is well suited to the use of numerous othertypes of adhesive well known in the art, including but not limited topotting epoxy. After curing of ribbon of B-staged epoxy 60, anintegrated-circuit die, not shown, is mounted on printed circuit board50 and electrical connection is made from bonding pads of theintegrated-circuit die to respective bonding fingers, typically shown as58, of leadframe 56. By placing ribbon of B-staged epoxy 60 on leadframe56, detachment of bonding fingers 58 from electrical contact regions 52of printed circuit board 50 is prevented, thereby improving thereliability of the semiconductor assembly.

Referring next to FIG. 6, a perspective view of another embodiment ofthe present claimed invention is shown. Although ribbon of B-stagedepoxy 60 is disposed over leadframe 56 only above the point at which theinner ends of bonding fingers 58 are attached to electrical contactregions 52 of printed circuit board 50 in FIG. 5, the present claimedinvention is well suited to other configurations of ribbon of B-stagedepoxy 60. As shown in the embodiment of FIG. 6, ribbon of B-staged epoxy60 is placed over the entire surface of leadframe 56, and printedcircuit board 50 except for the portion of printed circuit board 50which is exposed through the opening centrally located within leadframe56, and wire bond areas 54 of printed circuit board 50. In such aconfiguration, because ribbon of B-staged epoxy 60 is chemicallycompatible with the plastic material used in subsequent processes toencapsulate the semiconductor assembly, the adhesion of mold compound tothe semiconductor assembly is enhanced. Furthermore, a thermallyconductive ribbon of B-staged epoxy 60 may also be used in the presentclaimed invention to increase heat dissipation away from theintegrated-circuit die, not shown.

Thus, the present claimed invention provides a reinforced leadframe tosubstrate attachment which does not substantially increase the cost ofthe assembly and which is compatible with standard subsequent processingsteps.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical application,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the Claims appended hereto and theirequivalents.

I claim:
 1. A method of reinforcing attachment of a leadframe to aprinted circuit board, comprising the steps of:bonding inner ends ofbonding fingers of a leadframe having a plurality of inwardly-extendingbonding fingers disposed peripherally surrounding a centrally locatedopening within said leadframe to a printed circuit board havingelectrical contact regions formed thereon such that each of said innerends of said inwardly-extending bonding fingers is bonded to arespective electrical contact region on said printed circuit board,placing a layer of adhesive material only on said leadframe such thatsaid leadframe is disposed between said layer of adhesive and saidprinted circuit board and such that the bonding between the inner endsof the bonding fingers and the printed circuit board is reinforced bythe layer of adhesive material, curing said layer of adhesive material,mounting an integrated-circuit die having bonding pads formed thereon towire bond areas of said printed circuit board such that saidintegrated-circuit die is disposed within said centrally located openingin said leadframe and such that the layer of adhesive material bordersthe integrated-circuit die so that the layer of adhesive material doesnot cover the integrated-circuit die, and electrically coupling each ofsaid bonding pads on said integrated-circuit die electrically to arespective inner end of the bonding fingers.
 2. The method as recited inclaim 1 wherein said step of attaching said inner ends of the bondingfingers of the leadframe to a printed circuit board further comprisesattaching said inner ends to said printed circuit board by thermalcompression.
 3. The method as recited in claim 1 wherein said step ofplacing the layer of adhesive material on said leadframe furthercomprises placing a layer of epoxy on said leadframe.
 4. The method asrecited in claim 3 wherein said step of placing the layer of epoxy onsaid leadframe further comprises placing said layer of epoxy on saidleadframe only above where said inner ends of each of saidinwardly-extending bonding fingers of said leadframe are bonded to saidrespective electrical contact region on said printed circuit board. 5.The method as recited in claim 3 wherein said step of placing the layerof epoxy on said leadframe further comprises placing said layer of epoxyover said leadframe and said printed circuit board except for said wirebond areas of said printed circuit board and said centrally locatedopening in said leadframe when said leadframe is disposed on saidprinted circuit board.
 6. The method as recited in claim 1 wherein saidstep of placing the layer of adhesive on said leadframe furthercomprises placing a ribbon of B-staged epoxy on said leadframe.
 7. Themethod as recited in claim 6 wherein said step of placing the ribbon ofB-staged epoxy on said leadframe further comprises placing said ribbonof B-staged epoxy on said leadframe only above where said inner ends ofeach of said inwardly-extending bonding fingers of said leadframe arecoupled to said printed circuit board.
 8. The method as recited in claim6 wherein said step of placing the ribbon of B-staged epoxy on saidleadframe further comprises placing said ribbon of B-staged epoxy oversaid leadframe and said printed circuit board except for said wire bondareas of said printed circuit board and said centrally located openingin said leadframe when said leadframe is disposed on said printedcircuit board.
 9. The method as recited in claim 6 wherein said step ofplacing the ribbon of B-staged epoxy on said leadframe further comprisesplacing a ribbon of thermally conductive B-staged epoxy on saidleadframe.